Drug interactions are a little bit more complicated than simply the dosing times recommended on a box. Drugs have "half lives" and this is the amount of time that half the drug is metabolized. Perhaps half the drug is gone after 4 hours than another half in another 4 hours...this would mean 25% is still in the body.
That''s a good question for a doctor or your pharmacist. That probably won''t do any serious damage once or twice, but it may not be a good pattern to follow. It merits additional discussion with a professional.